There are at least two major techniques known in the art for coupling together the ends of two optical fibers to enable the laser beam being transmitted through one such fiber to efficiently enter and be transmitted by the other fiber. Each such technique is, however, to the extent the inventors herein are aware, limited to communication and low power applications in which the laser beam average power is substantially less than one watt. Such techniques for coupling fibers transmitting low power laser beams are unsuitable for high power laser beam applications. High power is used herein to refer to laser beams having an average power ranging from approximately one watt to hundreds of watts. The terms "power" and "high power" are used interchangeably herein to describe high power laser beams. Design considerations and different coupling techniques for coupling fibers transmitting low power laser beams are surveyed in the paper entitled "A Review of Optical Fiber Connection Technology" by Dalgleish, Proceedings of 25th International Wire and Cable Symposium, November 1976, pp. 240-246. The known coupling techniques and apparatus can, generally, be divided into two broad categories. The first category consists of splicing techniques in which the ends of the respective fiber cores to be coupled are abutted and then fused together. U.S. Pat. No. 4,263,495 to Fujita et al. discloses such a technique in which the fuming is achieved by irradiation of the abutting fiber ends with a CO.sub.2 laser. A technique for fiber end fusion by heating the abutting ends with an electric arc is described in the paper entitled "Fusion Splices in Single-Mode Optical Fibers" by Hatakeyama et al., IEEE Journal of Quantum Electronics, Vol. QE-14, August 1978, pp. 614-619. Such splicing methods result in average splicing losses that are apparently acceptable for communication or low power apparatus. Further, such techniques are only illustrated and described for practice on small diameter fibers, e.g. 10 micrometers or less, such fibers only being capable of single mode beam transmission. In contrast, fibers having diameters in excess of 100 micrometers are typically employed for the multimode transmission of power laser beams. There is no apparent suggestion or intention that the fibers spliced in accordance with the prior art splicing techniques are suitable for transmitting a high power laser beam. The inventors herein believe that based on the dB losses indicated for such splices, transmission of a power laser beam through the splice would result in heat generation and subsequent failure of the splice. Since no technique appears to exist for splicing together optical fibers of sufficiently large diameter and to achieve sufficiently low losses for power laser beam transmission, this technique is believed by the instant inventors to be unsuitable for power laser beam applications.
The second category of techniques for coupling two optical fibers consists of using a connector. In one type of connector, the respective end portions of the fibers being butt connected are each rigidly supported, e.g. in a close fitting bore, in respective connector halves. Then, when the two connector halves are fastened together, such as by screw engagement, the fiber ends are rigidly aligned and substantially abutting. The accuracy of the alignment of the respective fiber ends is dependent on the precision with which the connector is fabricated. In some such connectors, each fiber end may be supported by a metal or ceramic ferrule which is in turn supported within the connector body. Another type of connector comprises a single sleeve with a bore narrowing from both ends to guide into alignment the respective fiber cores being coupled. Such connectors typically include immersion of the respective end portions of the fiber cores in an index matching gel in order to minimize transmission losses. Neither of the above described types of connectors are available for high power laser beam applications with respect to either accommodating sufficient fiber diameter or laser power. With respect to the connector type comprising engaging halves, any misalignment would result in a stray portion of the power laser beam impinging on a portion of the connector body, and/or supporting ferrule, to cause heating. Such heating will at very least cause thermal distortion, if not physical damage, to the connector resulting in further misalignment. Further, fiber misalignment may result in thermally induced damage to the fiber end intended to receive the power laser beam. Additionally, as described in greater detail below and as known in the art, power laser beam injection into an optical fiber is most efficiently achieved where the beam is focussed onto the prepared fiber end in accordance with specific criteria. Thus, the mere abutting alignment of two fibers results in an extremely inefficient connection. The above described single sleeve connector type also suffers from the shortcomings of merely relying on end-to-end fiber alignment. Additionally, the inventors have observed that the index matching gels typically employed in the art are absorptive of energy carried in the power laser beam and, as a result, will burn causing connector failure.
In addition to the above described two broad categories of coupling techniques, the instant inventors are aware of a particular connector manufactured by Eastman Kodak Company as a LAMDEK single-mode connector. The connector employs two glass aspheric lenses to respectively collimate the laser beam emanating from one fiber end and focus the collimated beam onto the other fiber end. The end portion of each fiber is encased in a glass ferrule and each fiber tip and ferrule are affixed to the surface of its associated lens with an index matching cement. The LAMDEK connector appears not to be intended for use in high power laser beam applications and, in any event, is unsuitable for high power applications for several reasons. First, its single mode characteristic limits the fiber diameter that can be coupled to approximately 10 micrometers or less. Typical power laser beam applications require multimode beam transmission and employ fiber diameters on the order of from 100 to 1000 micrometers or larger. Second, the lenses are fabricated of glass and are therefore unsuitable for high power laser beam applications. It is the experience of the instant inventors that glass lenses thermally distort and crack upon subjection to even moderate beam power densities. Third, the fact that the ferrules are composed of glass renders them susceptible to fracture should a portion of the power laser beam impinge thereon due to misalignment. Fourth, the LAMDEK device depends on affixing, with cement, each fiber tip, and ferrule, to its associated lens to enable adjustable alignment between the fiber ends. It is the instant inventor's experience that, as with the above noted index matching gel, no cement will withstand the power densities experienced during high power laser beam transmission. Thus, since the alignment feature depends on the ability to affix fiber to lens, the LAMDEK connector is neither suitable for nor adaptable to high power laser beam applications.
It is therefore a principal object of the present invention to provide a coupling device for coupling high power laser beam transmitting optical fibers that is not subject to the aforementioned inadequacies of prior art coupling techniques and devices.